Polyurethane polishing pad

ABSTRACT

The polishing pad is suitable for planarizing at least one of semiconductor, optical and magnetic substrates. The polishing pad includes a cast polyurethane polymeric material formed with an isocyanate-terminated reaction product formed from a prepolymer reaction of a prepolymer polyol and a polyfunctional isocyanate. The isocyanate-terminated section product has 4.5 to 8.7 weight percent unreacted NCO; and the isocyanate-terminated reaction product is cured with a curative agent selected from the group comprising curative polyamines, curative polyols, curative alcoholamines and mixtures thereof. The polishing pad contains at least 0.1 volume percent filler or porosity.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of pending application U.S.Ser. No. 10/772,054, filed Feb. 3, 2004, now abandoned.

BACKGROUND

This specification relates to polishing pads useful for polishing andplanarizing substrates and particularly to polishing pads having uniformpolishing properties.

Polyurethane polishing pads are the primary pad-type for a variety ofdemanding precision polishing applications. These polyurethane polishingpads are effective for polishing silicon wafers, patterned wafers, flatpanel displays and magnetic storage disks. In particular, polyurethanepolishing pads provide the mechanical integrity and chemical resistancefor most polishing operations used to fabricate integrated circuits. Forexample, polyurethane polishing pads have high strength for resistingtearing; abrasion resistance for avoiding wear problems duringpolishing; and stability for resisting attack by strong acidic andstrong caustic polishing solutions.

The production of semiconductors typically involves several chemicalmechanical planarization (CMP) processes. In each CMP process, apolishing pad in combination with a polishing solution, such as anabrasive-containing polishing slurry or an abrasive-free reactiveliquid, removes excess material in a manner that planarizes or maintainsflatness for receipt of a subsequent layer. The stacking of these layerscombines in a manner that forms an integrated circuit. The fabricationof these semiconductor devices continues to become more complex due torequirements for devices with higher operating speeds, lower leakagecurrents and reduced power consumption. In terms of device architecture,this translates to finer feature geometries and increased metallizationlevels. These increasingly stringent device design requirements aredriving the adoption of copper metallization in conjunction with newdielectric materials having lower dielectric constants. The diminishedphysical properties, frequently associated with low k and ultra-low kmaterials, in combination with the devices' increased complexity haveled to greater demands on CMP consumables, such as polishing pads andpolishing solutions.

In particular, low k and ultra-low k dielectrics tend to have lowermechanical strength and poorer adhesion in comparison to conventionaldielectrics, rendering planarization more difficult. In addition, asintegrated circuits' feature sizes decrease, CMP-induced defectivity,such as, scratching becomes a greater issue. Furthermore, integratedcircuits' decreasing film thickness requires improvements in defectivitywhile simultaneously providing acceptable topography to a wafersubstrate—these topography requirements demand increasingly stringentplanarity, dishing and erosion specifications.

Casting polyurethane into cakes and cutting the cakes into several thinpolishing pads has proven to be an effective method for manufacturingpolishing pads with consistent reproducible polishing properties.Vishwanathan et al., in PCT Pub. No. 01.91971 disclose a set ofproperties for improving polishing performance including E′ (elasticstorage modulus) ratio at 30° C. and 90° C. and several otherproperties. Unfortunately, polyurethane pads produced from the castingand skiving method can have polishing variations arising from apolishing pad's casting location. For example, pads cut from a bottomcasting location and a top casting can have different densities andporosities. Furthermore, polishing pads can have center-to-edgevariations in density and porosity within a pad. These variations canadversely affect polishing for the most demanding applications, such aslow k patterned wafers. Thus, there is a demand for a polyurethanepolishing pad with improved density and porosity uniformity.

STATEMENT OF INVENTION

The invention provides a polishing pad suitable for planarizing at leastone of semiconductor, optical and magnetic substrates, the polishing padcomprising a cast polyurethane polymeric material formed from aprepolymer reaction of a prepolymer polyol and a polyfunctional aromaticisocyanate to form an isocyanate-terminated reaction product, thepolyfunctional aromatic isocyanate having less than 8 weight percentaliphatic isocyanate and the isocyanate-terminated reaction producthaving 4.5 to 8.7 weight percent unreacted NCO, theisocyanate-terminated reaction product being cured with a curative agentselected from the group comprising curative polyamines, curativepolyols, curative alcoholamines and mixtures thereof; and the polishingpad containing at least 0.1 volume percent filler or porosity.

In another aspect of the invention, the invention provides a polishingpad suitable for planarizing semiconductor substrates, the polishing padcomprising a cast polyurethane polymeric material formed from aprepolymer reaction of a prepolymer polyol selected from the groupcomprising polytetramethylene ether glycol, polyester polyols,polypropylene ether glycols, copolymers thereof and mixtures thereof anda polyfunctional aromatic isocyanate to form an isocyanate-terminatedreaction product, the polyfunctional aromatic isocyanate having lessthan 5 weight percent aliphatic isocyanate and the isocyanate-terminatedreaction product having 4.5 to 8.7 weight percent unreacted NCO, theisocyanate-terminated reaction product being cured with a curative agentwith expandable polymeric microspheres, the curative agent selected fromthe group comprising curative polyamines, curative polyols, curativealcoholamines and mixtures thereof; and the polishing pad containing aporosity of at least 0.1 volume percent.

In another aspect of the invention, the invention provides a method offorming a polishing pad suitable for planarizing semiconductorsubstrates comprising casting polyurethane polymeric material from aprepolymer reaction of a prepolymer polyol and a polyfunctional aromaticisocyanate to form an isocyanate-terminated reaction product, thepolyfunctional aromatic isocyanate having less than 8 weight percentaliphatic isocyanate and the isocyanate-terminated reaction producthaving 4.5 to 8.7 weight percent unreacted NCO, theisocyanate-terminated reaction product being cured with a curative agentselected from the group comprising curative polyamines, curativepolyols, curative alcoholamines and mixtures thereof; and the polishingpad containing at least 0.1 volume percent filler or porosity.

DETAILED DESCRIPTION

Cast polyurethane polishing pads are suitable for planarizingsemiconductor, optical and magnetic substrates. The pads' particularpolishing properties arise in part from a prepolymer reaction product ofa prepolymer polyol and a polyfunctional isocyanate. The prepolymerproduct is cured with a curative agent selected from the groupcomprising curative polyamines, curative polyols, curative alcoholamines and mixtures thereof to form a polishing pad. It has beendiscovered that controlling the amount of unreacted NCO in theprepolymer reaction product can improve porous pads' uniformitythroughout a polyurethane casting.

In particular, controlling the prepolymer's weight percent unreactedNCO, appears to limit the exotherm from the chain extension reaction.This limits the temperature increase within the cast material and canimprove the uniformity of density across pads and through the “as cast”cakes. The lower pad uniformity of earlier cast polyurethane polishingpads arises from the high weight percent NCO of Adiprene L325 (Adiprene®is a urethane prepolymer product of Crompton/Uniroyal Chemical) used toproduce IC™ pads from Rohm and Haas Electronic Materials CMPTechnologies. But because a large part of the available NCO in AdipreneL325 is the less reactive aliphatic 4,4′-dicyclohexylmethanediisocyanate rather than all TDI, the exotherm is not as large as itwould be with an all aromatic isocyanate system. Controlling theprepolymer reaction product's reactive weight percent NCO improves thetemperature uniformity during the manufacturing process by controllingthe exothermic heat of reaction. If the weight percent NCO is too high,then the polishing pad can overheat in the middle and top portions,especially for polishing pads skived from cast polyurethane cakes. Ifthe weight percent NCO is too low, then the polyurethane will have toolong of a gel time that can also lead to non-uniformity, such as, thesinking of high-density particles or floating of low-density particlesand pores during an extended gelation process. Controlling theprepolymer's weight percent unreated NCO to between 4.5 and 8.7 weightpercent provides cast polyurethane polishing pads with uniformproperties. Preferably, the prepolymer's weight percent unreacted NCO isbetween 4.7 and 8.5.

The polymer is effective for forming porous and filled polishing pads.For purposes of this specification, filler for polishing pads includesolid particles that dislodge or dissolve during polishing, andliquid-filled particles or spheres. For purposes of this specification,porosity includes gas-filled particles, gas-filled spheres and voidsformed from other means, such as mechanically frothing gas into aviscous system, injecting gas into the polyurethane melt, introducinggas in situ using a chemical reaction with gaseous product, ordecreasing pressure to cause disolved gas to form bubbles. The polishingpads contain a porosity or filler concentration of at least 0.1 volumepercent. This porosity or filler contributes to the polishing pad'sability to transfer polishing fluids during polishing. Preferably, thepolishing pad has a porosity or filler concentration of 0.2 to 70 volumepercent. Most preferably, the polishing pad has a porosity or fillerconcentration of 0.25 to 60 volume percent. Preferably the pores orfiller particles have a weight average diameter of 10 to 100 μm. Mostpreferably, the pores or filler particles have a weight average diameterof 15 to 90 μm. The nominal range of expanded hollow-polymericmicrospheres' weight average diameters is 15 to 50 μm.

Controlling the unreacted NCO concentration is particularly effectivefor controlling the pore uniformity for pores formed directly orindirectly with a filler gas. This is because gases tend to undergothermal expansion at a much greater rate and to a greater extent thansolids and liquids. For example, the method is particularly effectivefor porosity formed by casting hollow microspheres, either pre-expandedor expanded in situ; by using chemical foaming agents; by mechanicallyfrothing in gas; and by use of dissolved gases, such as argon, carbondioxide, helium, nitrogen, and air, or supercritical fluids, such assupercritical carbon dioxide or gases formed in situ as a reactionproduct.

For polishing pads containing gaseous pores or gaseous-filledmicrospheres, a polishing pad's non-uniformity appears to be driven bythe following: 1) the temperature profile of the reacting system; 2) theresulting pore expansion in areas where the temperature increases abovethat of the expansion temperature of the pore while the surroundingpolymeric matrix remains not-so-locked in place as to be able torespond; and 3) the viscosity profile of the reacting or solidifyingpolymer matrix as a result of reaction and various local heating andcooling effects. In the case of a pore added through polymeric hollowmicrospheres, their Tg is related to the threshhold temperature forresponse. Polymeric microspheres above this temperature tend to grow anddeform in shape. When casting with hollow polymeric microspheres andwith the controlled weight percent unreacted NCO, the microspheres'pre-casting volume and the microspheres' final volume preferably remainswithin 8 percent of the average pre-casting volume throughout the castpolyurethane material. Most preferably, the microspheres' final volumeremains within 7 percent of the pre-casting volume throughout the castpolyurethane material.

Literature shows a volume decrease as a function of time forpre-expanded Expancel microspheres maintained at elevated temperatures.However, the further expansion of the expanded microspheres contributesto increased non-uniformity of the polishing pads. By controlling thethermal history in the casting process by limiting weight percentunreacted NCO, polishing pads with more uniform density throughout bothindividual pads and the cake are produced. Pad formulations with moreuniform density can provide more consistent removal rates andtopographical control than pad formulations where this is uncontrolled,giving greater CMP process control in actual use.

With Adiprene L325 prepolymers, peak exotherm temperatures reach as highas 264° F. (129° C.). These temperatures are well above the expansiononset temperature and closely approach the temperatures of maximumexpansion for Expancel microsphere 551DU40—the unexpanded microspheresfrom which 551DE40d42 is produced—275-289° F. (135-143° C.). Typically,the density in the center of the cast cake is lower due to greaterheating and the resulting greater pore expansion. Polishing pads'porosity variation also tends to increase with increasing initial porevolume, increasing material temperatures and increasing mass of castmaterial.

Because the pore can only expand if the surrounding polymer is stillsufficiently mobile that it can rearrange with a small pressure, it isalso important that the weight percent unreacted NCO of the system andthe ability of the polymer backbone to order is not too low, or thepores or filler can slowly expand or segregate by density, yielding abroader density distribution.

Preferably, the polymeric material is a polyurethane. For purposes ofthis specification, “polyurethanes” are products derived fromdifunctional or polyfunctional isocyanates, e.g. polyetherureas,polyesterureas, polyisocyanurates, polyurethanes, polyureas,polyurethaneureas, copolymers thereof and mixtures thereof. An approachfor controlling a pad's polishing properties is to alter its chemicalcomposition. In addition, the choice of raw materials and manufacturingprocess affects the polymer morphology and the final properties of thematerial used to make polishing pads.

Preferably, urethane production involves the preparation of anisocyanate-terminated urethane prepolymer from a polyfunctional aromaticisocyanate and a prepolymer polyol. For purposes of this specification,the term prepolymer polyol includes diols, polyols, polyol-diols,copolymers thereof and mixtures thereof. Preferably, the prepolymerpolyol is selected from the group comprising polytetramethylene etherglycol [PTMEG], polypropylene ether glycol [PPG], ester-based polyols,such as ethylene or butylene adipates, copolymers thereof and mixturesthereof. Example polyfunctional aromatic isocyanates include 2,4-toluenediisocyanate, 2,6-toluene diisocyanate, 4,4′-diphenylmethanediisocyanate, naphthalene-1,5-diisocyanate, tolidine diisocyanate,paraphenylene diisocyanate, xylylene diisocyanate and mixtures thereof.The polyfunctional aromatic isocyanate contains less than 8 weightpercent aliphatic isocyanates, such as 4,4′-dicyclohexylmethanediisocyanate, isophorone diisocyanate and cyclohexanediisocyanate.Typically, aliphatic isocyanates are less reactive than aromaticisocyanates and release heat into the system more gradually. Preferably,the polyfunctional aromatic isocyanate contains less than 5 weightpercent aliphatic isocyanates and more preferably, less than 1 weightpercent aliphatic isocyanate.

Example prepolymer polyols include polyether polyols, such as,poly(oxytetramethylene)glycol, poly(oxypropylene)glycol and mixturesthereof, polycarbonate polyols, polyester polyols, polycaprolactonepolyols and mixtures thereof. Example polyols can be mixed with lowmolecular weight polyols, including ethylene glycol, 1,2-propyleneglycol, 1,3-propylene glycol, 1,2-butanediol, 1,3-butanediol,2-methyl-1,3-propanediol, 1,4-butanediol, neopentyl glycol,1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, diethyleneglycol, dipropylene glycol and mixtures thereof.

Preferably the prepolymer polyol is selected from the group comprisingpolytetramethylene ether glycol, polyester polyols, polypropylene etherglycols, polycaprolactone polyols, copolymers thereof and mixturesthereof. If the prepolymer polyol is PTMEG, copolymer thereof or amixture thereof, then the isocyanate-terminated reaction product mostpreferably has a weight percent unreacted NCO range of 5.8 to 8.7.Particular examples of PTMEG family polyols are as follows: Terathane®2900, 2000, 1800, 1400, 1000, 650 and 250 from DuPont; Polymeg® 2000,1000, 1500, 650 from Lyondell; PolyTHF® 650, 1000, 1800, 2000 from BASF,and lower molecular weight species such as 1,2-butanediol,1,3-butanediol, and 1,4-butanediol. If the prepolymer polyol is a PPG,copolymer thereof or a mixture thereof, then the isocyanate-terminatedreaction product most preferably has a weight percent unreacted NCOrange of 5 to 8. Particular examples of PPG polyols are as follows:Arcol® PPG-425, 725, 1000, 1025, 2000, 2025, 3025 and 4000 from Bayer;Voranol® 220-028, 220-094, 220-110N, 220-260, 222-029, 222-056, 230-056from Dow; Desmophen® 1110BD, Acclaim® Polyol 4200 both from Bayer If theprepolymer polyol is an ester, copolymer thereof or a mixture thereof,then the isocyanate-terminated reaction product most preferably has aweight percent unreacted NCO range of 4.5 to 7. Particular examples ofester polyols are as follows: Millester 1, 11, 2, 23, 132, 231, 272, 4,5, 510, 51, 7, 8, 9, 10, 16, 253, from Polyurethane Specialties Company,Inc.; Desmophen® 1700, 1800, 2000, 2001KS, 2001K², 2500, 2501, 2505,2601, PE65B from Bayer; Rucoflex S-1021-70, S-1043-46, S-1043-55 fromBayer.

Typically, the prepolymer reaction product is reacted or cured with acurative polyol, polyamine, alcohol amine or mixture thereof. Forpurposes of this specification, polyamines include diamines and othermultifunctional amines. Example curative polyamines include aromaticdiamines or polyamines, such as, 4,4′-methylene-bis-o-chloroaniline[MBCA], 4,4′-methylene-bis-(3-chloro-2,6-diethylaniline) [MCDEA];dimethylthiotoluenediamine; trimethyleneglycol di-p-aminobenzoate;polytetramethyleneoxide di-p-aminobenzoate; polytetramethyleneoxidemono-p-aminobenzoate; polypropyleneoxide di-p-aminobenzoate;polypropyleneoxide mono-p-aminobenzoate;1,2-bis(2-aminophenylthio)ethane; 4,4′-methylene-bis-aniline;diethyltoluenediamine; 5-tert-butyl-2,4- and3-tert-butyl-2,6-toluenediamine; 5-tert-amyl-2,4-and3-tert-amyl-2,6-toluenediamine and chlorotoluenediamine. Optionally, itis possible to manufacture urethane polymers for polishing pads with asingle mixing step that avoids the use of prepolymers.

The components of the polymer used to make the polishing pad arepreferably chosen so that the resulting pad morphology is stable andeasily reproducible. For example, when mixing4,4′-methylene-bis-o-chloroaniline [MBCA] with diisocyanate to formpolyurethane polymers, it is often advantageous to control levels ofmonoamine, diamine and triamine. Controlling the proportion of mono-,di- and triamines contributes to maintaining the cross-linking within aconsistent range. In addition, it is often important to controladditives such as anti-oxidizing agents, and impurities such as waterfor consistent manufacturing. For example, since water reacts withisocyanate to form gaseous carbon dioxide, controlling the waterconcentration can affect the concentration of carbon dioxide bubblesthat form pores in the polymeric matrix.

The polyurethane polymeric material is preferably formed from aprepolymer reaction product of toluene diisocyanate andpolytetramethylene ether glycol with 4,4′-methylene-bis-o-chloroaniline.Preferably, the prepolymer reaction product has a 4.55 to 8.7 weightpercent unreacted NCO. Examples of suitable prepolymers within thisunreacted NCO range include: Airthane® prepolymers PET-70D, PHP-70D,PET-60D, PET-95A, PET-93A, PST-95A, PPT-95A, Versathane® prepolymersSTE-95A, STE-P95, Versathane®-C prepolymers 1050, 1160, D-5QM, D-55, D-6manufactured by Air Products and Chemicals, Inc. and Adiprene®prepolymers LF600D, LF601D, LF700D, LF950A, LF952A, LF939A, LFG963A,LF1930A, LF1950A, LF1600D, L167, L367 manufactured by Uniroyal ChemicalProducts division of the Crompton Corporation. In addition, blends ofother prepolymers besides those listed above could be used to reach toappropriate %unreacted NCO levels as a result of blending. Many of theabove-listed prepolymers, such as, LF600D, LF601D, LF700D, and LFG963Aare low-free isocyanate prepolymers that have less than 0.1 weightpercent free TDI monomer and have a more consistent prepolymer molecularweight distribution than conventional prepolymers, and so facilitateforming polishing pads with excellent polishing characteristics. Thisimproved prepolymer molecular weight consistency and low free isocyanatemonomer give an initially lower viscosity prepolymer that tends to gelmore rapidly, facilitating viscosity control that can further improveporosity distribution and polishing pad consistency. For mostprepolymers, the low free isocyanate monomer is preferably below 0.5weight percent. Furthermore, “conventional” prepolymers that typicallyhave higher levels of reaction (i.e. more than one polyol capped by adiisocyanate on each end) and higher levels of free toluene diisocyanateprepolymer should produce similar results. In addition, low molecularweight polyol additives, such as, diethylene glycol, butanediol andtripropylene glycol facilitate control of the prepolymer reactionproduct's weight percent unreacted NCO.

In addition to controlling weight percent unreacted NCO, the curativeand prepolymer reaction product preferably has an OH or NH₂ to unreactedNCO stoichiometric ratio of 80 to 120 percent; and most preferably, ithas an OH or NH₂ to unreacted NCO stoichiometric ratio of 80 to 110percent.

If the polishing pad is a polyurethane material, then the polishing padpreferably has a density of 0.5 to 1.25 g/cm³. Most preferably,polyurethane polishing pads have a density of 0.6 to 1.15 g/cm³.

EXAMPLES

The following Table provides prepolymer and microsphere formulations forcasting polyurethane cakes. These formulations contained various amountsof polymeric microspheres for producing porosity with differentprepolymer formulations. These formulations tested toluene diiosocyanate[TDI] with polytetramethylene ether glycol [PTMEG], polypropylene etherglycol [PPG] and ester backbones from isocyanate-terminated prepolymers.As shown in following Tables, formulations 1 to 9 represent formulationsof the invention and formulations A to E represent comparative examples.In particular, comparative example A corresponds to the formulation ofExample 1 of U.S. Pat. No. 5,578,362; and comparative example Bcorresponds to the formulation of the IC1000™ polyurethane polishingpads sold by Rohm and Haas Electronic Materials CMP Technologies. Theamount of unreacted NCO contained in the isocyanate-terminatedprepolymers range from 5.3 to 9.11 percent.

TABLE 1 Polishing Pad Ingredients Unreacted Estimated Polyol IsocyanateNCO Microsphere Microsphere Microsphere Formulation Backbone ADIPRENEWt. % EXPANCEL Wt. % Density (g/cc) A-1 PTMEG L325 9.11 551DE40d42 1.780.043 A-2 PTMEG L325 9.11 N/A 0.00 0.043 B-1 PTMEG L325 9.11 551DE40d421.58 0.043 B-2 PTMEG L325 9.11 551DE40d42 2.10 0.043 B-3 PTMEG L325 9.11551DE40d42 1.56 0.043 B-4 PTMEG L325 9.11 N/A 0.00 0.043 B-5 PTMEG L3259.11 551DE40d42 1.58 0.043 C-1 PTMEG LF751D 9.02 N/A 0.00 0.042 C-2PTMEG LF751D 9.02 551DE40d42 0.89 0.042 C-3 PTMEG LF751D 9.02 551DE40d421.71 0.042 D PTMEG LF600D 7.12 N/A 0.00 0.042 1 PTMEG LF600D 7.12551DE40d42 0.88 0.042 2 PTMEG LF600D 7.12 551DE40d42 1.75 0.042 E PTMEGLF700D 8.13 N/A 0.00 0.042 3 PTMEG LF700D 8.13 551DE40d42 0.87 0.042 4PTMEG LF700D 8.13 551DE40d42 1.73 0.042 5 PTMEG LF600D 7.18 551DE40d421.25 0.043 6 PTMEG LF950A 5.99 551DE40d42 2.01 0.042 7-1 PTMEG LF950A5.99 551DE20d60 1.76 0.060 7-2 PTMEG LF950A 5.99 551DE20d60 1.78 0.055 8PPG LFG963A 5.75 551DE40d42 1.25 0.043 9-1 Ester LF1950A 5.4 551DE20d602.56 0.060 9-2 Ester LF1950A 5.3 551DE20d60 2.55 0.061 Adiprene ® is aurethane prepolymer product of Crompton/Uniroyal Chemical. L325 is aH₁₂MDI/TDI - PTMEG having an unreacted NCO of 8.95 to 9.25 wt %. LF600Dis a TDI - PTMEG having an unreacted NCO of 7.1 to 7.4 wt %. LF700D is aTDI - PTMEG having an unreacted NCO of 8.1 to 8.4 wt %. LF751D is aTDI - PTMEG having an unreacted NCO of 8.9 to 9.2 wt %. LF950A is aTDI - PTMEG having an unreacted NCO of 5.9 to 6.2 wt %. LFG963A is aTDI-PPG having an unreacted NCO of 5.55 to 5.85 wt %. LF1950A is aTDI-ester having an unreacted NCO of 5.24 to 5.54 wt %. Expancel ®551DE40d42 is a 30-50 μm weight average diameter hollow-polymericmicrosphere manufactured by Akzo Nobel Expancel ® 551DE20d60 is a 15-25μm weight average diameter hollow-polymeric microsphere manufactured byAkzo Nobel N/A = Not Applicable

The microspheres represent hollow or gas-filled polymeric spheresexpanded from other Expancel® microspheres. Table 2 below provides theexpansion onset and expansion maximum temperatures for the microspheresbefore expansion.

TABLE 2 Microsphere Expansion Temperatures Density MicrosphereSpecification Expanded from Expansion Expansion Expansion Expansion(Expanded) Range g/liter Microsphere Onset T, ° F. Onset T, ° C. Max T,° C. Max T, ° C. 551DE20d60 55 to 65 551DU20 199-210 93-98 264-279129-137 551DE40d42 38 to 46 551DU40 199-210 93-98 275-289 135-143

The polymeric pad materials were prepared by mixing various amounts ofisocyanate-terminated-urethane prepolymers with4,4′-methylene-bis-o-chloroaniline [MBCA] at the prepolymer temperaturesand MBCA temperatures provided in Table 3. At these temperatures, theurethane/polyfunctional amine mixture had a gel time on the order of 4to 12 minutes after adding of hollow elastic polymeric microspheres tothe mixture. The 551DE40d42 microspheres had a weight average diameterof 30 to 50 μm, with a range of 5 to 200 μm; and the 551DE20d60microspheres had a weight average diameter of 15 to 25 μm, and wereblended at approximately 3,600 rpm using a high shear mixer to evenlydistribute the microspheres in the mixture. The final mixture wastransferred to a mold and permitted to gel for about 15 minutes.

The mold was then placed in a curing oven and cured with a cycle asfollows: thirty minutes ramped from ambient temperature to a set pointof 104° C., fifteen and one half hours at 104° C. (except comparativeexamples A-1 and A-2 where this segment is changed to 5 h hours at 93°C.) and two hours with a set point reduced to 21° C. The molded articlewas then “skived” into thin sheets and macro-channels or grooves weremachined into the surface at room temperature—skiving at highertemperatures may improve surface roughness.

TABLE 3 Casting Conditions MBCA Prepolymer Prepolymer flow MBCA ExpancelPour Cake Cake Main oven Time at main flow rate, temperature, rate,temperature, flow rate, time, Diameter, Height, temperature,temperature, Formulation kg/min ° F./° C. g/min ° F./° C. g/min minutesin./cm in./cm ° F./° C. hours A-1 3.00 122/50 770 240/116 68.3 3 26/662/5.1 200/93  5 A-2 3.00 122/50 770 240/116 0 3 26/66 1/2.5 200/93  5B-1 4.15 123/51 1040 241/116 83.26 3 26/66 2/5.1 220/104 15.5 B-2 3.10122/50 780 240/116 83.14 4 36/91 1/2.5 220/104 15.5 B-3 5.00 125/52 1250240/116 99.3 4 34/36 2/5.1 220/104 15.5 B-4 4.15 123/51 1040 240/116 0 326/66 1.5/3.8   220/104 15.5 B-5 4.15 124/51 1040 241/116 83.56 3 26/662/5.1 220/104 15.5 C-1 4.61 127/53 1233 243/117 0 3 26/66 2/5.1 220/10415.5 C-2 4.62 126/52 1238 244/118 52.5 4 36/91 1.5/3.8   220/104 15.5C-3 4.61 128/53 1230 243/117 101.5 4 36/91 1.5/3.8   220/104 15.5 D 4.63126/52 989 244/118 0 3 26/66 2/5.1 220/104 15.5 1 4.63 127/53 999245/118 50 4 36/91 1.5/3.8   220/104 15.5 2 4.62 128/53 1001 243/117100.1 4 36/91 1.5/3.8   220/104 15.5 E 4.62 127/53 1117 243/117 0 326/66 2/5.1 220/104 15.5 3 4.62 126/52 1117 242/117 50.2 4 36/911.5/3.8   220/104 15.5 4 4.61 125/52 1109 242/117 100.4 4 36/911.5/3.8   220/104 15.5 5 4.15 123/51 850 240/116 63.5 3 26/66 2/5.1220/104 15.5 6 4.15 121/49 710 240/116 99.82 3 26/66 2/5.1 220/104 15.57-1 4.15 119/48 710 240/116 87.12 3 26/66 2/5.1 220/104 15.5 7-2 4.15123/51 710 233/112 88.04 3 26/66 2/5.1 220/104 15.5 8 4.15 123/51 800241/116 62.88 3 26/66 2/5.1 220/104 15.5 9-1 4.15 135/57 640 239115126.02 3 26/66 2/5.1 220/104 15.5 9-2 4.15 140/60 640 237/114 125.15 434/36 1.25/3.2   220/104 15.5

The following Table compares calculated density with actual top paddensity for the prepolymer formulations containing controlled amounts ofunreacted NCO. Predicted density and actual top pad density begin todeviate more significantly for comparative examples C-2 and C-3 that usean all TDI, high percent unreacted NCO prepolymer and a large molddiameter—all factors tending to increase product non-uniformity.

TABLE 4 Density Variation Unreacted Theor Predicted Actual Top NCOMicrosphere Theor Vol/g Theor Vol/g Vol/g Density Pad DensityFormulation Wt. % Wt. % Urethane Microsphere Mixture g/cc g/cc A-1 9.111.78 0.843 0.413 1.256 0.796 0.790 A-2 9.11 0.00 0.858 0.000 0.858 1.1651.165 B-1 9.11 1.58 0.843 0.366 1.209 0.827 0.826 B-2 9.11 2.10 0.8380.487 1.325 0.755 0.734 B-3 9.11 1.56 0.843 0.363 1.206 0.829 0.826 B-49.11 0.00 0.856 0.000 0.856 1.168 1.168 B-5 9.11 1.58 0.843 0.368 1.2100.826 0.827 C-1 9.02 0 0.841 0.000 0.841 1.189 1.189 C-2 9.02 0.89 0.8330.211 1.045 0.957 0.895 C-3 9.02 1.71 0.827 0.407 1.233 0.811 0.727 D7.12 0 0.856 0.000 0.856 1.169 1.169 1 7.12 0.88 0.848 0.210 1.058 0.9450.955 2 7.12 1.75 0.841 0.417 1.257 0.795 0.794 E 8.13 0 0.836 0.0000.836 1.196 1.196 3 8.13 0.87 0.829 0.207 1.035 0.966 0.946 4 8.13 1.730.822 0.411 1.233 0.811 0.783 5 7.18 1.25 0.845 0.292 1.137 0.880 0.8806 5.99 2.01 0.839 0.479 1.318 0.759 0.795 7-1 5.99 1.76 0.841 0.2941.134 0.882 0.874 7-2 5.99 1.78 0.841 0.324 1.164 0.859 0.837 8 5.751.25 0.859 0.291 1.150 0.870 0.871 9-1 5.4 2.56 0.755 0.427 1.183 0.8460.841 9-2 5.3 2.55 0.755 0.417 1.173 0.853 0.852 The formulation 8calculation uses Uniroyal's Adiprene LFG963A S.G. of 1.15 for unfilledmaterial The formulation 9 calculation uses Uniroyal's Adiprene LF1950AS.G. of 1.29 for unfilled material

Table 4 shows a general correlation between top pad density and thepredicted pad density.

Table 5 contains the maximum exotherm temperature obtained for castingeach polyurethane cake.

TABLE 5 Maximum Exotherm Temperature Unreacted NCO Exotherm max,Exotherm max, Formulation Wt % ° F. ° C. B-1 9.11 257 125 B-5 9.11 258126 5 7.18 235 113 6 5.99 215 102 7-2 5.99 209 98 8 5.75 163 73 9-1 5.4230 110

The above Table illustrates that controlling the unreacted NCO to lessthan 9.1 facilitates limiting the exotherm temperature to below 120° C.

A series of density measurements taken from top, middle and bottom padscompared through-cake uniformity of 80 mil (2 mm) polishing pads. Theaverage density represents the center, edge and midpoint density forpads from the three cake locations. In addition, the center, edge andmidpoint densities represent the average of four measurements.

TABLE 6 Density Uniformity Avg Density Unreacted through Cake NCOMicrospheres Exotherm Exotherm Cake Diameter, Formulation Wt. % Wt. %max, ° F. max, ° C. g/cc St Dev in./cm A-1 9.11 1.78 ND ND 0.785 0.02026/66 B-1 9.11 1.58 257 125 0.818 0.012 26/66 B-2 9.11 2.10 ND ND 0.7410.030 36/91 5 7.18 1.25 235 113 0.877* 0.003 26/66 6 5.99 2.01 215 1020.781 0.006 26/66 8 5.75 1.25 163 73 0.865 0.010 26/66 ND = NotDetermined *Determined by measuring entire pads through the cake.

These data indicate that an unreacted NCO range can improve the densitystandard deviation for cast polishing pads.

Because the amount of pad material contacting the surface to be polishedis related to the density of the pad material, polishing performancemeasures such as removal rates and topographical control are expected tobe greatly influenced by the density of a particular formulation. Ascontrol of polishing performance is driven to ever tighter requirementsby smaller linewidths and more fragile wafer materials, the importanceof improving the control of pad characteristics becomes increasinglyimportant. The porous-polyurethane polishing pads cast with a prepolymerhaving a controlled amount of unreacted NCO show a smaller standarddeviation for density measurements both across a pad and through a cake.

1. A method of forming a polishing pad suitable for planarizingsemiconductor substrates comprising the steps of: casting polyurethanepolymeric material, a curative agent and expandable polymericmicrospheres in a mold of a cake, the polyurethane polymeric materialbeing from a prepolymer reaction of a prepolymer polyol and apolyfunctional aromatic isocyanate to form an isocyanate-terminatedreaction product, the polyfunctional aromatic isocyanate having lessthan 1 weight percent aliphatic isocyanate and less than 0.5 weightpercent free isocyanate monomer, the isocyanate-terminated reactionproduct having 4.5 to 8.7 weight percent unreacted NCO and an OH or NH₂to unreacted NCO stoichiometric ratio of 80 to 110 percent, and theexpandable polymeric microspheres containing pre-expanded or expanded insitu microspheres; gelling the isocyanate-terminated reaction productwith a gel time sufficient to control porosity distribution from theexpandable polymeric microspheres in the mold; limiting exotherm of thecast isocyanate-terminated reaction product to a temperature below 120°C. to control density uniformity of the expandable polymericmicrospheres within polishing pads from the cake and maintain averagevolume of the expandable polymeric microspheres in the polishing padswithin 7 percent of average volume of the expandable polymericmicrospheres before casting; curing the gelled isocyanate-terminatedreaction product with the curative agent and the expandable polymericmicrospheres within the isocyanate-terminated reaction product to formthe cake, the curative agent being selected from the group comprisingcurative polyamines, curative polyols, curative alcoholamines andmixtures thereof; and skiving the cake to form the polishing pads; andthe polishing pads containing a porosity of 0.1 to 70 volume percentfrom the expandable polymeric microspheres, the expandable polymericmicrospheres having a weight average diameter of 10 to 100 μm and thepolishing pads having a density of 0.6 to 1.15 g/cm³.
 2. The method ofclaim 1 wherein the polishing pads include the expandable polymericmicrospheres having a concentration of 0.25 to 60 volume percent and theexpandable polymeric microspheres are pre-expanded.
 3. The method ofclaim 1 wherein the polishing pads include the expandable polymericmicrospheres having a weight average diameter of 15 to 50 μm.
 4. Amethod of forming a polishing pad suitable for planarizing semiconductorsubstrates comprising the steps of: casting polyurethane polymericmaterial, a curative agent and expandable polymeric microspheres in amold of a cake, the polyurethane polymeric material being from aprepolymer reaction of a prepolymer polyol and a polyfunctional aromaticisocyanate to form an isocyanate-terminated reaction product, thepolyfunctional aromatic isocyanate having less than 1 weight percentaliphatic isocyanate and less than 0.5 weight percent free isocyanatemonomer, the isocyanate-terminated reaction product having 4.5 to 8.7weight percent unreacted NCO and an OH or NH₂ to unreacted NCOstoichiometric ratio of 80 to 110 percent, and the expandable polymericmicrospheres containing pre-expanded or expanded in situ microspheres;gelling the isocyanate-terminated reaction product with a gel timesufficient to control porosity distribution from the expandablepolymeric microspheres in the mold; limiting exotherm of the castisocyanate-terminated reaction product to a temperature below 120° C. tocontrol density uniformity of the expandable polymeric microsphereswithin the mold of the cake and maintain average volume of expandablepolymeric microspheres in polishing pads from the cake to a volumewithin 7 percent of average volume of the expandable polymericmicrospheres before casting; curing the gelled isocyanate-terminatedreaction product with the curative agent and the expandable polymericmicrospheres within the isocyanate-terminated reaction product to formthe cake, the curative agent being selected from the group comprisingcurative polyamines, curative polyols, curative alcoholamines andmixtures thereof; and skiving the cake to form the polishing pads; andthe polishing pads containing a porosity of 0.1 to 70 volume percentfrom the expandable polymeric microspheres, the expandable polymericmicrospheres having a weight average diameter of 10 to 50 μm and thepolishing pads having a density of 0.6 to 1.15 g/cm³.
 5. The method ofclaim 4 wherein the polishing pad includes the expandable polymericmicrospheres having a concentration of 0.25 to 60 volume percent and theexpandable polymeric microspheres are pre-expanded.
 6. The method ofclaim 4 wherein the polishing pad includes the expandable polymericmicrospheres having a weight average diameter of 15 to 50 μm.